Concrete Foundations
A Concrete Placing Guide
Cast Your foundations
This guide will help owner-builders to cast the foundations. These foundations are for external brick walls around a new house, or for extensions of an existing house.
THE NHBRC (National Homebuilders Association Council) requirements for house foundations:
For single-story buildings with tiled or sheeted roofs in stable soil conditions, the minimum width of strip foundations should be 500mm for external walls and 400mm for internal walls. If a reinforced concrete roof is required on the house then the minimum width is 750mm for external walls and 600mm for internal walls.
Trenches
Trenches on sloping ground can be stepped with a minimum of 400mm overlap. Sloped trenches must not exceed the 1:10 gradient.
The minimum depth of the strip foundation trench must be at least 400mm below the original ground level.
The average minimum thickness of the foundation must be at least 200mm. The strength of the concrete used for unreinforced strip footing must be at least 15MPa or better. For a double-story building reinforced foundations would require 25–30MPa.
Trench sides
Trench sides and walls must be as near to vertical as possible. Trim the sides of the excavation to avoid holes being filled with concrete on the side faces. This will require more concrete than estimated and will be a waste.
When you have finished the excavation measure the actual depth, width, and length of the trench, and calculate the actual volume of concrete that you will need. Always add an additional 10% to allow for wastage and unforeseen circumstances. See our tables here: Concrete mixes by weight & volume
Placing
When placing your concrete there are a few things to consider.
Clear the soil that was excavated well away from the edges of the trench to stop it from falling back into the trench and contaminating the concrete. If you are using wheelbarrows to ferry the cement then place planks along the edges where there is soft bedding such as fill material to prevent loose soil from falling into trenches due to the weight of the wheelbarrows.
Gently wet the excavation trenches before placing the concrete to avoid moisture being drained out of the concrete mix into the surrounding soil as this could weaken the foundation.
Compact the concrete to remove any air bubbles. This will increase the density and strength.
Curing
Fresh concrete needs to be protected from drying out too quickly. Gently spray water over the foundation as soon as the concrete has set. In hot dry areas, the concrete can be covered with plastic sheeting to stop the concrete from drying too quickly. Remove the plastic and spray water onto the concrete every so often to prevent it from drying too quickly. When concrete loses moisture too quickly either by evaporation or absorption by the surrounding soil, it shrinks and if the concrete is not malleable, it will crack. See our article on drying in cold weather.
Here is an extract about water and concrete from a Wiki article:
The most destructive agent of concrete structures and components is probably water. Indeed, water often directly participates in chemical reactions as a reagent and is always necessary as a solvent, or a reacting medium, making the transport of solutes and reactions possible. Without water, many harmful reactions cannot progress or are so slow that their harmful consequences become negligible during the planned service life of the construction. Dry concrete has a much longer lifetime than water-saturated concrete in contact with circulating water. So, when possible concrete must first be protected from water infiltrations.
How soon can bricklaying start on the foundation?
Forty-eight hours after placing the concrete. You can build the first 500mm of brickwork above the surrounding ground level. Then you must carefully wet the concrete thoroughly and backfill your trenches with soil. Seven days after placing the concrete foundation strips, they should be strong enough and your walls can be built up to roof level. At this stage, the concrete should have reached 60% of the final strength.
Read more about concrete:
https://cemcon-sa.org.za/information-hub/information-centre/
Hi Pete, For 40×40×2 mm galvanized steel posts, 1200 mm high and spaced 1200 mm apart, a typical foundation size would be around 250–300 mm diameter and 500–600 mm deep, depending on soil conditions. In Rivonia’s generally stable soils, 600 mm depth should provide adequate embedding. Make sure the posts are well anchored with concrete, and consider wind load if they support fencing or panels.
What size foundations and embedment depth would you recommend for 40 x 40 x 2 mm galvanised steel posts, 1200 mm high off the ground and spaced 1200 mm c-c, to be installed in Rivonia, Johannesburg?
Thank you for your question! Planning ahead for future construction is a smart approach, and reinforcing foundations for a potential double-story house is a crucial step to ensure your home can safely bear the additional load. Here’s how you can proceed:
1. Consult a Structural Engineer
The first step is to consult with a structural engineer who will assess the current soil conditions, the existing house design, and the potential load of a future double-story. They will provide you with detailed plans and specifications for the foundation reinforcement. This is a critical step, as the design of foundations for a double-story differs significantly from a single-story.
2. Conduct a Soil Test
A geotechnical survey or soil test is required to determine the bearing capacity of the soil. This will help the engineer design foundations that are suitable for your soil type and ensure the structure remains stable over time.
3. Design Stronger Foundations
Foundations for a double-story home generally need to be deeper and wider than those for a single-story. Key considerations include:
Depth of foundations: Deeper footings are required to support the additional weight.
Width of foundations: Wider footings distribute the load more effectively.
Steel reinforcement: Additional steel reinforcement will be required in both the foundation and the walls.
Your structural engineer will specify the exact dimensions and materials needed to ensure the foundations are suitable for future expansion.
4. Ensure Compliance with Building Regulations
The design and construction must comply with the National Building Regulations and Building Standards Act (NBR), SANS 10400. These regulations dictate the minimum requirements for foundations, walls, and other structural elements, especially for double-story buildings. The reinforced foundations should be included in the approved house plans.
5. Build with Future Expansion in Mind
When building your single-story house:
Columns: Consider installing reinforced concrete columns or “stub columns” in key load-bearing areas that can support the weight of the second story later.
Walls: Use materials and wall thicknesses suitable for a double-story, even for the single-story phase.
Roof Design: The roof structure should allow for easy removal or modification when adding the second story.
6. Submit Updated Plans to the Municipality
If your current plans only cater to a single-story house, you will need to update them to include the reinforced foundations for a double-story and submit them to your local municipality for approval. The updated plans must clearly show the structural details and reinforcement.
7. Cost Considerations
Reinforcing your foundations now will likely increase your initial construction costs, but it will save you significant expense and effort when you decide to build the second story. It’s far easier and more cost-effective to reinforce foundations during the initial construction phase than to try and retrofit them later.
8. Avoid Retrofitting Later
If foundations are not reinforced during the initial construction, adding a second story later could result in:
Major structural challenges.
High costs for underpinning or foundation reinforcement.
Potential risk of structural failure.
By planning now, you ensure safety, compliance, and reduced costs in the future.
By following these steps, you’ll be well-prepared to expand your home to a double-story in the future. If you need further assistance, don’t hesitate to consult your local municipality or a qualified structural engineer. Good luck with your project!
Is one layer of 9mm mesh enough in one storey house strip foundation or must be double layered, with spacers/stools between the two?
Hi, Please help me pass the NHBRC exam, I read the NHBRC Guide and SANS 10400 could not give me most of the answers to pass the exam
I just want to say your answers are very informative and direct… Thank you
You don’t say how far off ground level the existing floors are. But since the previous owner has successfully filled beneath the floor of three rooms, do what they did. I assume they would need to be filled with concrete to create a slab. You should use a registered builder to do the job, and possibly consult with an engineer.
Hello Please advise I have an old house with suspended floors not many people can give me advise. I’m at my wits end with these suspended floors. 3 rooms of the original structure have already been filled by the previous owner
Currently 3 rooms and the passage still have suspended floors I want to fill these floors and I need advise and competent person that could do it for me please assist
I don’t agree as long come in a rural part of and my grandfather’s home is still standing,has more than 90 years and guess what, no foundation was used.the materials were cow dug mix with soil and wood.so just because we’ve been introduced to new technology we must not ever forget our old invention…..
I don’t agree as long come in a rural part of and my grandfather’s home is still standing,has more than 90 years and guess what, no foundation was used.the materials were cow dug mix with soil and wood.so just because we’ve been introduced to new technology we must not ever forget our old invention…..
Hi,
Does the trench size and footingvthickness differ for a three storey block of flats, or just the strength of the cement mix?
Thanks
Rose
Annelie, There is a misconception in SA that if you live on a farm or small holding you can build without plan approval. The law states that all buildings in SA have to have plans approved by the nearest local planning department. If it is a new house you will have to register with the NHBRC or get an exemption from them as an owner builder. The house has to conform to the SANS10400 building standards. The SANS10400 has no standard for a plastic bottle house building method. Any non-standard construction method needs to have approval from the SABS. The best advice is to contact an architect who knows what the process is and get his help for your approval. Another bit of advice here is that if you do build illegally and later you want to sell the new buyer will need plans especially if they apply for a bond from a bank. You will be in for penalties and you will have to do plans anyway.
Hi. I am planning to build a plastic bottle house like this one: https://diyprojects.ideas2live4.com/2015/10/02/how-to-build-a-house-using-plastic-bottles/. I want to know what the specs are for the foundation I will need to build. The walls of these structures are extremely strong on its own but I know I still need a foundation. Any help here would be appreciated because I would like it all to be legal and get my certificate even if it will be built on a plot or farm.
Please advise
I have single story house plan what must I do to re inforce fou dario to accommodate double story later on in life
Lucas I am not available to answer calls telephonically. If you have questions please ask them here.
Hi Penny. Can you please give me your nr. I have a coyple questions i want to ask you.
No structure should be built without concrete foundations! Chances are if weather conditions are bad (e.g. heavy rain or strong wind) the house might collapse and kill you! You also need plans that should be approved by the local authority!